Electrolytic migration in metals has been a problem in microelectronic packaging for years and is becoming more threatening as line spacings become smaller and electronic fields larger. Thus as the microelectronic arts progress, the conductor spacings on the dielectric substrates decrease. The resultant decreased spacings between the leads enhances electrolytic migration which is manifested by the formation of dendrites that act to short parallel or substantially parallel adjacent leads held at different electrical potentials.
The basic requirements for the formation of dendrites are the presence of two differently biased metal electrodes, an electrolyte and time. These factors in combination form an electrolytic cell such that under bias, the metal of the lead goes into solution at an anode (i.e. a first lead) in the form of metal ions, and the resultant ions migrate through the electrolyte to a cathode (i.e. a second lead) where they are reduced and deposited. This problem is particularly significant for a system comprising copper leads disposed on a polymer substrate.
In such a system, the polymer substrate absorbs and adsorbs water from the atmosphere. This water, in combination with ionic residue (from manufacturing or other processing steps) present on the polymer substrate surface, forms an electrolyte. A differential bias between the copper leads results in electrolytic copper plating that forms dendrites at the cathode with the result that after a relatively short period of time, a bridge of dendrites forms between the leads thereby resulting in a short.
Electrolytic migration and the corresponding formation of dendrites is known to occur very rapidly for silver and less rapidly for many other metals including copper. Electrolytic migration can be a problem with gold as well, although halide ions seem to be an additional requirement for the formation of gold dendrites.
At one time, it was believed that corrosion-type problems relating to copper leads could be prevented by coating the leads in a package with a thin layer (e.g., about 0.6 .mu.m) of gold. However, even in such packages which have been coated with a thin layer of gold, dendrites are still formed as a result of copper dissolving from poorly coated areas or gold dendrites are formed in the presence of chloride ions.
Another especially troublesome problem associated with microelectronic packaging system is the delamination of the copper leads from the substrate. In the event of delamination of the copper lead, the copper on the underside of the lead provides a large source of metal ions for electrolytic migration.
One alternative preventive method which has been used, is to coat the entire package with a non-porous dielectric material. This coating is intended to prevent the adsorption and absorption of the moisture between the leads, and to provide a physical barrier to dendrite formation. A serious drawback to this approach is that it relies on a "zero-defect" criterion which means that any delamination or pin holes in the coating could cause failure.
The prior art discloses the use of the individual metal layers disclosed in the present invention alone and in combination, however the composition and arrangement of specific metal layers of the present invention are not disclosed.
For example, U.S. Pat. No. 4,323,060 discloses a method of electrolessly depositing nickel or cobalt, including nickel containing phosphorous over a copper conductor on a board. Gold can be deposited on the nickel coated copper line to provide for corrosion resistance or to enhance the line solderability.
Likewise, U.S. Pat. No. 4,503,131 discloses depositing nickel or nickel-phosphorous between a copper conductor and a top layer of gold to provide enhanced corrosion resistance and to meet industry contact resistance standards under severe exposure conditions.
U.S. Pat. No. 4,717,591 is typical of the metal coating disclosures in that it discloses a coating of nickel-phosphorous on to copper leads.
There is no disclosure in the representative prior art references relating in general to the subject matter of the instant invention that discloses or suggests the composition or structure of the present invention.